It's actually the other way around. The growth plates stay open longer when a dog is neutered before puberty, so the dog can get slightly taller and lankier than when they're done at 6 months or more. But the differences are so slight, I doubt anybody would be able to tell.

Some people say that Boo has the typical body of a dog neutered earlier and would have been stalkier had I done him later, but he was done later. I only got him at around 6 months old and we had to wait until his kennel cough was gone and the dispute with the SPCA over the neutering was over too.